To determine the effect of porcelain thickness and the application of a surface liner (SL) on the flexural strength ( σ ) of a ceramic system.
One hundred twenty bar-shaped specimens of yttria-stabilized zirconia-based ceramic were fabricated and randomly divided into two groups according to the application of SL: A – applied; B – not applied. The specimens were further divided according to the porcelain veneer thickness ( n = 20): 0–0.5 mm; 1–1.0 mm; 2–2.0 mm. All specimens were tested in 3-point bending with the porcelain under tension. The maximum load was recorded at first sign of fracture, the σ was calculated and the mode of failure was determined. Data was statistically analyzed using one-way ANOVA, Tukey ( α = 0.05) and Weibull distribution.
The SL application had no effect ( p = 0.723) while the porcelain thickness had a significant effect ( p = 0.000) on the mean σ values. No significant differences in mean σ values were found with same porcelain thickness. A 0.5-mm thick porcelain layer (groups A0 and B0) showed greater mean σ values than other groups. The Weibull modulus ( m ) ranged from 6 (groups A1 and B2) to 9 (groups A0, A2 and B0). The predominant mode of failure for all groups was fracture initiation at the porcelain surface propagating to the ceramic interface.
The porcelain thickness influenced the mean σ values, but the SL had no significant effect on the flexural strength and the mode of failure of the ceramic system examined.
The transformation toughened zirconia, potentially the most interesting polycrystalline ceramic available for dentistry, represents a great advance in dental ceramics, mainly because its fracture toughness involves an additional strengthening mechanism not found in other polycrystalline ceramics . The zirconium oxide, in normal conditions of pressure, is transformed from a crystalline state to another depending on temperature. Zirconia is cubic at temperatures above 2370 °C, tetragonal from 1170 to 2370 °C, and monoclinic under 1170 °C, which means that at firing temperatures (app. 1370 °C) zirconia is tetragonal, changing to monoclinic at room temperature . An increase of cell volume (3–4%) comes with the transformation from tetragonal to monoclinic form, which could lead to the formation of microcracks throughout the material on cooling. The addition of small amounts of oxides (3–8 mass%), such as CaO, MgO, Y 2 O 3 , and CeO 2 , to the ceramic composition metastabilizes the tetragonal form at room temperature, controlling the stress induced by the transformation . Tetragonal crystals present in the structure at room temperature are able to transform back to the monoclinic state in response to stress such as the highly localized stress in the vicinity of a propagating crack tip. In this case the increase in volume becomes beneficial, essentially representing a force opposing to the crack opening and decreasing the local stress intensity .
The transformation toughening mechanism provides zirconia with the characteristics required for a dental prostheses substructure material, such as high flexural strength (in the range of 800–1000 MPa) and fracture toughness (from 6 to 8 MPa m 1/2 ) . Even though it has a certain level of translucency (semi-translucency), most dental zirconia are opaque because of their high crystalline content . Therefore, especially in esthetic areas of the mouth, the opacity of zirconia limits its use to the fabrication of substructures, which are further veneered with high-translucent dental porcelains.
Porcelain compatibility is a concern on veneered-zirconia restorations. Recent studies on ceramic restorations reported a significant amount of porcelain chipping (15–62%), cracking (25–50%), delaminations (less than 10.7%) and large fractures (3–33%) . Several potential explanations for such fracture behavior have been reported, including: residual stresses related to the ceramics thermal history (such as thermal expansion mismatches, sintering temperature, auto-catalytic transformation during porcelain firing and cooling rate) , restoration’s geometric factors (i.e., framework design, core–veneer thickness ratio and ceramic strength) , porcelain inherent deficiency in strength and porcelain bond to the zirconia substrate . The fact is that veneer fracture is a real clinical concern and it is the subject of comprehensive investigations, but still remains to be overcome.
Despite the fact that manufacturers aim to prevent thermal residual stresses providing core and veneering ceramics with compatible coefficient of thermal expansion (CTE), residual stresses may be accumulated in the structure during processing, especially at the veneering ceramic and its interface with the core . In general, thermal compatibility or incompatibility is determined based only on visible crack formation.
Analytical calculations and physical testing demonstrated that the thickness of the veneering ceramic layer plays an important role in the accumulation of stresses in the structure. It was shown that a thick veneer fired over a low thermal conductivity material, such as zirconia, is prone to generate high tensile stresses within the porcelain layer and at the interface with the core. Although such stresses may result in veneer premature failure in some clinical situations, the thickness of a ceramic restoration has to be excessively increased to achieve adequate shape and occlusion contacts, and therefore this parameter need to be better understood .
Residual tensile stress at core–veneer interface that do not result in visible crack formation might induce restoration premature fracture when the all-ceramic system is subjected to load . As an attempt to improve the contact between the materials at the interface and to avoid premature failure, some manufacturers recommend the use of surface liners (SL) fired over the zirconium dioxide substructures prior to the porcelain application. The effect of this intermediate layer on all-ceramic restorations fracture behavior remains unclear.
The objective of this study was to evaluate the effect of the porcelain veneer thickness and the application of a liner material on the flexural strength ( σ ) of an all-ceramic system, testing the hypotheses that (1) the flexural strength ( σ ) increases as the veneer layer decreases and (2) the liner layer promotes greater σ values.
Materials and methods
The chemical composition and manufacturers of the materials used in this study are presented in Table 1 .
|Brand name||Chemical composition a||Manufacturer|
|Lava Frame Zirconia (LYZ)||ZrO 2 (95%), Y 2 O 3 (<3%), HfO 2 (<1–3%), Al 2 O 3 (<1%) e SiO 2 (<1%).||3M-ESPE, St. Paul, MN, USA|
|Vita VM9 Effect Bonder (SL)||Powder: SiO 2 (47–51%), Al 2 O 3 (10–15%), K 2 O (5–8%), Na 2 O (3–5%), CeO 2 (10–13%), ZrO 2 (5–8%), CaO (1–2%), B 2 O 3 (3–5%), BaO (0.5–1.5%), TiO 2 (<0.5%), SnO 2 (<0.5%), Mg, Fe and P oxides (<0.1%) Liquid: containing ethanol (2.5–10%) e sodium hydroxide (2.5%)||Vita Zahnfabrik, Bad Sackingen, Germany|
|Vita VM9 Base Dentin (V9)||SiO 2 (60–64%), Al 2 O 3 (13–15%), K 2 O (7–10%), Na 2 O (4–6%), TiO 2 (<0.5%), CeO 2 (<0.5%), ZrO 2 (0–1%), CaO (1–2%), B 2 O 3 (3–5%), BaO (1–3%), SnO 2 (<0.5%), Mg, Fe and P oxides (<0.1%)||Vita Zahnfabrik, Bad Sackingen, Germany|
Non-sintered LYZ blocks were sectioned using a diamond disc in a cutting machine (Labcut 1010, Extec Corp., Enfield, USA). One hundred and twenty bar-shaped specimens were obtained and sintered (Lava™ Furnace 200, 3M-ESPE, St. Paul, MN, USA) according to the manufacturer instructions. The specimen dimensions (20 mm × 4.0 mm × 0.7 mm) were checked with a digital caliper (Starrett 797, L.S. Starrett Co., Athol, MA, USA).
Prior to veneering, the LYZ bars were sonically cleaned (Vitasonic, Vita Zanhfabrik, Bad Sackingen, Germany) in distilled water for 10 min and let dry. The SL was applied on the surface of sixty randomly selected LYZ specimens (group A). The liner was sintered (Vita Vacumat 40, Vita-Zahnfabrik, Germany) following the manufacturer instructions. The remaining sixty specimens (group B) received no other surface treatment before veneering but sonically cleaned.
The specimens in groups A and B were randomly divided into 3 sub-groups ( Table 2 ) according to the thickness of the porcelain (V9) applied on the LYZ bars.
|Group ( n = 20)||Liner application||Veneer thickness (mm). Target values||Total thickness (mm). a Average of measured values (SD)|
The LYZ specimens were placed into a stainless steel mold that allowed for veneer thickness control. The porcelain (V9) powder and liquid (Vita Modeling Liquid, Vita Zahnfabrik, Germany) were mixed and applied on the specimens’ surface. Excess liquid was blot out with absorbent paper, the specimen was extruded from the mold and sintered (Vita Vacuumed 40, Vita Zahnfabrik, Germany) following manufacture’s recommendations. All specimens were polished using 600, 800 and 1200-grit SiC metallographic paper (3M, St. Paul, MN, USA) and 1 μm diamond paste, assuring the homogeneity of the porcelain layer and avoiding any significant effect from porcelain thickness. The specimens’ edges were chamfered using a holding device following the ISO 6872:2008 standard recommendations. The final dimensions of all specimens ( Table 2 ) were examined with a digital caliper in 3 different locations (left end, mid-point and right end).
All specimens were submitted to a 3-point flexural strength test having the porcelain side under tensile stresses. The load was applied by a universal testing machine (DL-1000, EMIC, São José dos Pinhais, PR, Brazil) at the midpoint between the support rollers (cross-head speed = 0.5 mm/min). The maximum load ( P , in N) was recorded at first sign of fracture verified by sound emission and a change in the stress vs strain curve. The flexural strength ( σ f , in MPa) was calculated using the following equations :
σ f = 6 M w t v 2 K 2 + t c t v + E v t v E c t c
where M and K are obtained by the Eqs. (2) and (3) :
M = P L 4
K = 4 + 6 t c t v + 4 ( t c t v ) 2 + E c E v ( t c t v ) 3 + E v t v E c t c